The Universal Mobile Telecommunication System (UMTS) is one of the third generation mobile communication technologies designed to succeed GSM. 3GPP Long Term Evolution (LTE) is a project within the 3rd Generation Partnership Project (3GPP) to improve the UMTS standard to cope with future requirements in terms of improved services such as higher data rates, improved efficiency, and lower costs. The Universal Terrestrial Radio Access Network (UTRAN) is the radio access network of a UMTS system and evolved UTRAN (E-UTRAN) is the radio access network of an LTE system. As illustrated in FIG. 1, an E-UTRAN typically comprises user equipment (UE) 150 wirelessly connected to radio base stations (RBS) 100a-c, commonly referred to as eNodeB (eNB). The eNB controls one or more areas referred to as cells 110a-c. In FIG. 1 the UE 150 is served by the serving cell 110a. Cells 110b and 110c are neighboring cells.
Mobile user positioning is the process of determining UE coordinates in space. Once the coordinates are available, the position can be mapped to a certain place or location. The mapping function and the delivery of the location information on request are parts of the location service which is required for the basic emergency services. Services that further exploit the location knowledge, or that are based on location knowledge to offer customers some additional value, are referred to as location-aware and location-based services, respectively. The possibility of identifying a radio device's geographical location in the network has enabled a large variety of commercial and non-commercial services e.g. navigation assistance, social networking, location-aware advertising, and emergency calls. Different services may have different positioning accuracy requirements imposed by the application. Furthermore, regulatory requirements on the positioning accuracy for basic emergency services exist in some countries.
There exist a variety of positioning techniques in wireless communications networks, differing in their accuracy, implementation cost, complexity, and applicability in different environments. Positioning methods can be broadly categorized into satellite based and terrestrial methods. Global Navigation Satellite System (GNSS) is the standard generic term for satellite navigation systems that enable subscribers to locate their position and acquire other relevant navigational information. The global positioning system (GPS) and the European Galileo positioning system are well known examples of GNSS. In many environments, the position can be accurately estimated by using positioning methods based on GPS. Nowadays the networks also often have a possibility to assist UEs in order to improve the terminal receiver sensitivity and the GPS start up performance (Assisted-GPS positioning, or A-GPS). GPS or A-GPS receivers, however, are not necessarily available in all wireless terminals, and not all wireless networks have the possibility to provide or assist GPS-based positioning. Furthermore, GPS-based positioning may often have unsatisfactory performance in urban and/or indoor environments. There is thus a need for a complementary terrestrial positioning method implemented in the wireless network.
There are a number of terrestrial positioning methods, which determine the UE position by signals measured by the UE and/or by the radio network nodes such as the RBS, including:                cell identity based methods,        network based uplink time difference of arrival (U-TDOA) of signals at different RBS,        UE-based and UE-assisted observed time difference of arrival (OTDOA) of signals from three or more sites or locations, and        fingerprinting or the pattern matching positioning method.OTDOA is a positioning method which exploits the multi-lateration technique to calculate the UE position based on TDOA measurements from three or more locations. To enable positioning, the UE should thus be able to detect signals from at least three geographically dispersed RBS. This implies that the signals need to have high enough signal-to-interference ratios (SINR). Furthermore, the signals need to be transmitted frequently enough to meet the service delay requirements. In such a UE assisted solution, a serving mobile location centre (SMLC in GSM and UMTS, enhanced SMLC (E-SMLC) in LTE) calculates the UE position based on the positioning measurements reported by the UE. The E-SMLC 100 is either a separate network node (as illustrated in FIG. 1) or an integrated functionality in e.g. the eNB.        
OTDOA positioning is using Reference Signal Time Difference (RSTD) measurements as specified in the 3GPP standard i.e. the relative timing difference between the timing of a neighbour cell and a reference cell. In a positioning report containing measurements for a number of cells, a same reference cell is used for all of the measurements. The reference cell is one of the strongest cells and very often it is the UE's serving cell, although it might sometimes be another neighbour cell. This may for example be the case when a UE cannot handover to a stronger but overloaded cell or when the UE is close to a base station serving only UEs within a closed subscriber group.
Downlink UE-assisted OTDOA is currently being discussed in 3GPP for LTE, for which it has been commonly recognized that the hearability issue needs to be addressed to enable positioning service that meets the service requirements. It has e.g. been shown that using synchronization signals (SS) and cell-specific reference signals (CRS) for positioning without interference management results in positioning coverage problems due to low SINR and/or insufficient number of strong signals from different RBS. The problem is particularly relevant for synchronized networks or networks with high data load, as there is a high probability of parallel transmissions in multiple cells on the resource elements used for CRS or SS, which leads to high interference. To address these issues and enhance positioning measurements, new physical signals, positioning reference signals (PRS), in combination with low-interference subframes have been proposed. PRS are transmitted in downlink according to a predefined PRS pattern which may differ by e.g. site, cell, or subframe. This implies that the UE needs to know what PRS pattern that is used for a certain cell, in order for it to be able to detect the PRS and perform the positioning measurements needed for the positioning services. Positioning methods in E-UTRAN can apply so called Physical Cell Identities (PCI) as defined in the 3GPP standard. 3GPP defines 504 unique PCIs, which are intended to support efficient radio terminal UE measurement reporting procedures. The PCIs are grouped into 168 unique PCI groups, each group containing three unique identities. The grouping is such that each PCI is part of one and only one PCI group. A PCI calculated as NIDcell=3NID(1)+NID(2) is thus uniquely defined by a number NID(1) in the range of 0 to 167 representing the PCI group, and a number NID(2) in the range of 0 to 2 representing the PCI within the PCI group.
The number of available unique PCIs in LTE will most likely not be enough in typical radio network deployments and therefore have to be reused, so they can be viewed as local cell identities. A network, however, will also maintain unique global cell identifiers. PCIs are planned during the network planning phase, either during initial network planning or during network re-planning and optimization, and are physically assigned to the cells as part of the initial cell configuration procedure. There exists a relation between PCIs and physical signals, which means that PCI planning has a strong impact, for example, on physical-layer procedures like the cell search performed during the initial network access or at handovers.
As already mentioned above, the candidate signals for downlink positioning measurements in LTE are the SS, the CRS, and the PRS.
Secondary Synchronization Signals (SSS):
These signals, which are followed by the primary synchronization signal (PSS) are transmitted in subframes 0 and 5 in the second last OFDM (Orthogonal Frequency Division Multiplexing) symbol of the first time slot in the indicated subframes and only in 62 resource elements, i.e. over less than six Physical Resource Blocks (PRBs) in the centre of the allocated bandwidth. The mapping between PCI and SS is described in the 3GPP standard. To ensure good cell detection performance, PCIs have to be planned to avoid the same PSS and either of the two SSS short codes in common in neighbouring cells, utilizing 3 and 168 unique sequences available for PSS and SSS, respectively. In LTE there exists a mapping between SS sequences and PCI, but not between the SS transmission pattern and PCI though, as the pattern is always the same. However, because of bad cross-correlation properties (e.g. compared to CRS) due to short sequences, infrequent transmission, and a small transmission bandwidth, SSS are not optimal candidates for positioning measurements.
Cell-Specific Reference Signals (CRS):
These signals are transmitted on resource elements in every subframe, and over the entire bandwidth. Up to six frequency shifts are possible, which are specified as a function of the PCI k=6m+(v+vshift)mod 6, where v={0,3,6} (specified in 3GPP TS 36.211) and vshift=NIDcell mod 6. This corresponds in practice to a frequency re-use of six and three, for one and two (or four) transmit antennas respectively. The CRS transmission pattern is thus retrievable for a given PCI since essentially the same pattern is used applying different shifts in time (when there are four antenna ports) and frequency.
Positioning Reference Signals (PRS):
As mentioned above, PRS are newly proposed physical signals with transmission patterns that are different from those for the standardized physical signals (e.g. SSS and CRS). Careful planning of PRS patterns among the cells in order to minimize the interference on colliding resource elements in neighboring cells is crucial for the positioning performance. PRS patterns can generally be grouped into three groups:                Regular patterns—an example of a regular pattern is illustrated in FIG. 2(a)        Random patterns        Patterns based on Latin squares, Costas arrays and Modular sonar sequences. An example of a 9×9 Costas array is illustrated in FIG. 2(b).        
In general, given a set of patterns, it should be possible to group them such that all patterns within a group are orthogonal to each other, although they overlap in one or more resource elements with at least one pattern from any other group. A considerable disadvantage with random (pseudo-random) patterns is that large pattern tables may be required, which makes them less favourable for PRS since this may increase the signalling overhead and the RBS and UE complexity.
Mobility for a UE includes idle mode mobility (by means of cell reselection) and connected mode mobility (by means of handover). Cell reselection is a mainly UE autonomous function without any direct intervention of the network. However, to some extent the UE behaviour during the idle mode mobility scenario could still be controlled by broadcasted system parameters and a performance specification. Handover on the other hand is fully controlled by the network through explicit UE specific commands and by a performance specification. In both idle and connected modes the mobility decisions are mainly based on the same kind of downlink neighbour cell measurements. For example in E-UTRAN the following downlink neighbor cell measurements (measured for the serving and the neighbor cells) are specified primarily for mobility purpose:                Reference signal received power (RSRP), which is a cell-specific signal strength metric.        Reference signal received quality (RSRQ), which is a cell-specific signal quality metric, corresponding to RSRP/carrier RSSI (Received Signal Strength Indicator). RSSI is the total power received by the UE from all sources, including interfering sources, which means that the RSRQ also takes the interference into account.        
RSRP and RSRQ indicate the radio conditions experienced by the UE. A lower reported value of RSRP from the serving cell will e.g. depict that the UE is far from the serving base station.
A neighbour cell list (NCL) contains, for example, cell-specific information for neighbour cells and their physical cell identities (PCI). An NCL is typically created for mobility purposes. In e.g. UTRAN, an NCL is uploaded in each NB, and cell identities comprised in the NCL are measured to support the mobility decisions. The drawback of the NCL is that it requires extensive work to keep it updated, since a change in the cell planning also affects the NCL.
In LTE, the NCL is not mandatory for mobility. The UE instead blindly detects cell identities during the cell search procedure. However, in the existing positioning solution in LTE, the information related to the PRS patterns used in the cells is signalled to the UE in the form of an NCL. A similar solution exists in the UTRAN for positioning. However the NCL is error prone, and—as already explained above—expensive to dynamically maintain and update. It is a complex task to create an NCL with cell identities that are suitable for positioning measurements, since an optimal list may depend on the UE position in the cell. Furthermore, NCLs created for mobility are not very well suited for positioning since it is important that positioning measurements are done in cells located in different sites. Therefore, NCLs for mobility and positioning are not recommended to be used interchangeably. Therefore, even if there is an NCL signaled to the UE for positioning, the UE may need to create an additional cell list for mobility purpose.